This type of device, typically comprising a projection unit which produces a light beam designed to be directed towards a diffractive combiner for the purpose of projecting images, particularly information on functioning or driving of the vehicle, in the form of a virtual image situated in the driver's field of vision, was initially produced using technology derived from aeronautical applications. The manufacturing costs are therefore often considerable, liable to prevent them from being marketed and installed on a larger scale on lower cost vehicles.
These costs result in particular from the technology used, which is complicated to put into practice, and which further does not always allow mass replication of the combiners with sufficient guarantee of stability of optical characteristics. This is for example the case with the device disclosed in document JP10048562, describing a system which includes a hologram with index modulation, the manufacture of which is based on the use of photosensitive plates made from a gelatin-based photosensitive layer deposited on a substrate acting as a mechanical support. Such a component can be manufactured only singly, because it involves individualized recording, and it is not capable of mass manufacture at a reasonable industrial cost. The holographic component produced on this type of photosensitive plate is furthermore sensitive to UV radiation, which is capable of altering it, unless protective layers are added. This is also the case with the system disclosed in document EP0467328, showing a combiner with volumetric optical treatment manufactured on the basis of gelatins, and requiring several holographic layers interacting with several wavelengths, which makes reliable reproduction of the optical characteristics of the system even more uncertain. This system further works with reflected light only.
A system which functions with reflected light with several wavelengths is also described in document U.S. Pat. No. 4,930,847, with a recording method using both different geometry and wavelength, based on gelatin-type photosensitive materials which have the limitations set forth above.
Another structure with at least two layers is described in U.S. Pat. No. 6,005,714, where the multiplication of layers increases the difficulty of maintaining, in mass manufacture, correct stability of the optical function to be provided. The multilayer combiner with multilevel diffractive structures described in this document is manufactured by calculation carried out by computer, and suffers from the processing limitations of computers, allowing only reduced combiner dimensions for a finite number of levels of diffractive structures, which make it possible only to approximate the desired optical functions.
Manufacture resulting from calculations by computer is also described in document WO2004/09090607 which proposes a diffractive element which diffracts the light in several orders, superimposing the zero order of diffraction of ambient light on the first order of diffraction, and in particular requires a complex optical system of reproduction.